7.22 How are hardware devices made tamper-resistant?

There are many techniques that are used to make hardware tamper-resistant (see Question 7.21). Some of these techniques are intended to thwart direct attempts at opening a device and reading information out of its memory; others offer protection against subtler attacks, such as timing attacks and induced hardware-fault attacks.

At a very high level, a few of the general techniques currently in use to make devices tamper-resistant are:

Employing sensors of various types (for example, light, temperature, and resistivity sensors) in attempt to detect occurrences of malicious probing.

Packing device circuitry as densely as possible (dense circuitry makes it difficult for attackers to use a logic probe effectively).

Using error-correcting memory.

Making use of non-volatile memory so that the device can tell if it has been reset (or how many times it has been reset).

Using redundant processors to perform calculations, and ensuring that all the calculated answers agree before outputting a result.